1. Field of the Invention
The present invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of making a T-gate LDD pocket device in the fabrication of integrated circuits.
2. Description of the Prior Art
FIG. 1 illustrates a typical lightly doped drain (LDD) device of the prior art. Gate electrode 16 has been formed over gate silicon oxide layer 14 on the surface of semiconductor substrate 10. Lightly doped regions 20, heavily doped source and drain regions 22, and P+ regions 24 have been ion implanted into the semiconductor substrate at the gate edges.
There are a number of problems with this type of device structure. One problem is that the gate edge is stressed by high electrical field-induced leakage. Also, the thin gate oxide at the gate edge over the source/drain region induces a large capacitance during device operation. This thin gate oxide is easily damaged by electro-static discharge (ESD) at the N-region, and the high heat at the junction edge melts or otherwise adversely affects thin gate silicon oxide and polysilicon. The P+ pocket implantation after the N-implantation degrades the doping concentration of the N- region and reduces channel resistance, saturation current, and device operation speed. An improved LDD pocket device without the aforementioned problems is desired.
When the prior art MOS FET devices are used for ESD protection circuit purposes, these devices are easily damaged at the N- region gate oxide. The prior art gate over the source/drain region is too large. The N- implant will diffuse sideways by process heating, so the gate easily overlaps the N- region. The ESD will cause damage and device failure at the drain edge where the gate silicon oxide will break down due to the high heat developed by the ESD effects.
U.S. Pat. No. 5,108,937 to Tsai et al shows the formation of FOX structures and the removal of some of the FOX structures followed by formation of MOS FET devices. Large tilt angle implantation methods are generally known in the art.